This section illustrates useful background information without admission of any technique described herein being representative of the state of the art.
Fabry-Perot interferometers (FPI) are typically used as optical filters and in spectroscopic sensors, for example. The Fabry-Perot interferometer (FPI) is based on parallel mirrors, wherein a (Fabry-Perot) cavity is formed into a gap between the mirrors.
The pass band wavelength of a Fabry-Perot interferometer (FPI) can be controlled by adjusting the distance between the mirrors i.e. the width of the gap. The tuning is usually made electrically.
Microelectromechanical technology may be used for producing electrically tunable Fabry-Perot interferometers (FPI). Prior art structure of a microelectromechanical interferometer usually includes layers of silicon, wherein electrically conductive layers and reflective layers are doped. A movable mirror is provided by removing a sacrificial layer of silicon dioxide, which layer has initially been formed between two mirror layers. The position of a movable mirror is controlled by applying voltage to electrodes, which are included in the mirror structures.
The microelectromechanical production technology allows series production of interferometers. However, there are some disadvantages related with the prior art solutions for production of interferometers and the interferometer components.
Known solutions utilize silver-coated mirrors within Fabry-Perot interferometers (FPI). Also, the cutting, encapsulating and transportation of the interferometers require special handling because of the movable, released mirror. A released mirror is sensitive to environmental stress, such as physical pressure, changes of temperature or humidity, contamination, etc.
Due to the relatively high production cost of interferometers, it has not been possible to use them in mass product applications where the cost requirements are strict.
A further disadvantage of the known solutions relates to the inability to provide a gap with short distance between the mirrors. This is due to wet etching process where providing narrow gaps is difficult. Also, when Fabry-Perot interferometers (FPI) are produced for visible and ultraviolet light, the optical layers need to be thin. Thin membranes are often discontinuous and include pinholes. Such membranes easily become damaged during wet etching. Therefore the prior art technology is not suitable for producing electrically tunable Fabry-Perot interferometers (FPI) for short wavelengths such as visible and ultraviolet range.
It is the aim of the current invention to provide a method and apparatus that mitigates for example the above problem of the state of the art.